The present invention concerns capacitors, particularly those for use in medical devices, such as implantable defibrillators.
Every year more than half a million people in the United States suffer from heart attacks, more precisely cardiac arrests. Many of these cardiac arrests stem from the heart chaotically twitching, or fibrillating, and thus failing to rhythmically expand and contract as necessary to pump blood. Fibrillation can cause complete loss of cardiac function and death within minutes. To restore normal heart contraction and expansion, paramedics and other medical workers use a device, called a defibrillator, to electrically shock a fibrillating heart.
Since the early 1980s, thousands of patients prone to fibrillation episodes have had miniature defibrillators implanted in their bodies, typically in the left breast region above the heart. These implantable defibrillators detect onset of fibrillation and automatically shock the heart, restoring normal heart function without human intervention. The typical implantable defibrillator includes a set of electrical leads, which extend from a sealed housing into the heart of a patient after implantation. Within the housing are a battery for supplying power, heart-monitoring circuitry for detecting fibrillation, and a capacitor for storing and delivering a burst of electric charge through the leads to the heart.
The capacitor is typically an aluminum electrolytic capacitor. This type of capacitor usually includes stacked strips of aluminum foil and paper rolled up to form a cylindrical structure called an active element. The active element is typically placed in a round tubular can which is sealed shut with a flat circular lid, known as a header. (The header usually consists of two thin bonded layers, one rubber and the other phenolic resin.) Extending from the header are two terminals connected to the rolled up foils in the active element. The terminals are usually fastened to the lid using two rivets.
Each rivet has a short shank, or rod, with a broad head on one end. (The rivet head, typically round like the head of a nail, has a diameter of about four millimeters (three sixteenths of an inch) and a thickness of about one millimeter.) The shank extends through holes in the terminal and the header, with the head resting against the interior side of the header and its opposite end extending from the exterior side. The opposite end is flattened or otherwise deformed to prevent the shank from passing back through its hole, thereby fastening the terminal to the header.
In recent years, manufacturers of electrolytic capacitors have focused almost single-mindedly on improving the active element by developing aluminum foils, electrolytes, and multiple-anode arrangements that improve capacitor performance, specifically energy densityxe2x80x94the amount of energy or charge a capacitor stores per unit volume. For example, because energy density is directly proportional to the surface area of the aluminum foil making up the active element, manufacturers have developed methods of etching microscopic hills and valleys into foil to increase its effective surface area.
In comparison, capacitor manufacturers have made little or no effort to reduce the size of capacitors through space-saving assembly techniques. For example, the inventors determined that the conventional use of rivets to fasten terminals to the capacitor lid, or header, wastes space. Specifically, they determined that conventional capacitor manufacturers generally increase capacitor height (or reduce foil dimensions) to accommodate the heads of the rivets that fasten terminals to headers. The rivet heads are electrically conductive and must be prevented from touching, or contacting, the foils in the active element. So, capacitor manufacturers increase the height of the case to provide clearance between the rivet heads and the foils. Unfortunately, this increases not only the size of the capacitors, but also the size of devices, such as implantable defibrillators, that incorporate them.
Accordingly, the inventors identified an unmet need to reduce the size of electrolytic capacitors, especially those intended for implantable defibrillators, through better techniques and structures for fastening terminals to capacitor headers.
To address this and other needs, the inventors devised a capacitor having a header which includes one or more recesses. The recess receives the head of a rivet or other fastener and thus reduces or eliminates the need to increase capacitor height or reduce foil dimensions to achieve clearance between the fasteners and other capacitor parts, such as active-element foils. More particularly, the exemplary embodiment includes a header having two recesses, each with a depth that allows the head of a rivet to be substantially flush, or coplanar, with the underside of the header. In another embodiment, the header has a single recess to receive two rivet heads.
In devising this improvement, the inventors departed from at least two conventional capacitor design objectives: reducing the number of assembly steps per capacitor and reducing manufacturing waste or cost. Conventional capacitor manufacturers make hundreds of thousands or even millions of capacitors every year and are thus continually seeking ways to reduce capacitor assembly time. Indeed, saving (that is, omitting or skipping) even one manufacturing step amounts to considerable time and cost savings when multiplied by hundreds of thousands or millions of capacitors. Conversely, adding a step, such as forming one or more recesses in a header, to the manufacture of each capacitor generally increases assembly time and cost.
Similarly, conventional capacitor manufacturers who make thousands or millions of capacitors may also be concerned about reducing material waste, particularly seeking and developing capacitor designs and assembly practices which minimize or reduce the risk of destroying an entire capacitor or capacitor part during manufacture. Indeed, designs and manufacturing steps which pose a high risk of destroying an entire capacitor or capacitor part, such as a header, are generally avoided. Conventional headers are only about 2.5 millimeters thick and comprise two bonded layers of material. Forming one or more recesses in this type header not only adds a step to the manufacturing process, but also presents a risk of destroying it and thus increasing manufacturing waste and cost.